This invention relates to an adjustable load support for the lifting, lowering, and transfer of heavy loads. Such supports find particular use in connection with the movement of structural frameworks at construction sites, especially bridge construction sites.
Adjustable load supports of the type hereinafter described are typically constructed with a hollow container or receptacle. The container is usually made of steel and is either circular or rectangular in shape. A covering plate, also of steel, sits atop the container and protrudes partly into the interior of the container. The cover plate can be moved upwards or downwards. The cover plate together with the side walls of the container and its bottom form an enclosed pressurized compartment.
Inside the pressurized compartment, there is a mass of elastomeric material which substantially fills the interior of the compartment. This elastomeric mass is capable of deforming itself elastically when it is subjected to pressure. Special sealing elements are arranged on the underside of the cover plate near the region where the cover plate meets the side walls. Alternatively, the special sealing elements may be located in a channel of the elastomeric mass, said channel being located near the region where the cover plate meets the side walls. The purpose of the special sealing elements is to prevent the elastomeric material from being trapped inside the gap between the cover plate and the side walls when the elastomeric mass is subjected to high pressure. The sealing elements also prevent escape of the elastomeric material into the exterior of the pressurized compartment when it is subject to high pressure.
This type of load support has the additional feature that the cover plate on which the structural framework or bridge part rests can be tilted up to a certain angle with the horizontal. This can be accomplished by compressing the elastomeric mass on one side only. The covering plate will then incline itself at an angle without generating excessive friction because of the elasticity and flexibility of the edge sealing elements.
Load supports of this type can also be used for lifting and lowering heavy loads, such as structural frameworks for buildings or bridges, when they are provided with means for introducing a pressurized medium into the hollow compartment. When this is done, the pressurized medium exerts a force on the elastomeric mass causing the volume enclosed within the compartment to increase so that the cover plate bearing the structural framework lifts upwards. When the pressurized medium is removed from the compartment, the enclosed volume is reduced and the cover plate with the structural framework atop is lowered.
Height-adjustable load supports of this type are useful in many different kinds of tasks. They are useful in the construction of bridges or other heavy objects where one must contend with a sinking subsoil or foundation. Such sinking can be compensated for by elevating the load supports. When the sinking is too great and the load supports cannot be elevated sufficiently, the load supports must be removed and the base underneath must be raised. In instances such as this, adjustable load supports are used to raise the structural framework or the bridge so that previous load supports can be removed. It may also be necessary to raise the load in order to inspect the supports underneath or to change them. In such cases as well, adjustable load supports are needed to lift and then lower the structural framework or the bridge. Depending on the circumstances, it may suffice to build in new adjustable load supports and remove the old supports.
Adjustable load supports which have been used in the past have had problems associated with them. For example, in German Patent DE-PS No. 1759032, an adjustable load support is described wherein a pressurized medium is introduced into the pressurized compartment below the elastomeric mass via a hydraulic pressure line at the bottom of the pressurized compartment. In this patent, the pressurized medium is a plastic or elastic material which is forced into the hollow compartment on the bottom of the compartment and below the elastomeric material. This causes the cover plate to be displaced upwards. Thus, there is formed below the elastomeric mass a more or less uniform layer of the new material which has been forced in via the pressure line. Because of the non-compressibility of the elastomeric mass, the volume within the pressurized compartment is enlarged by the material which is forced into it, whereby the cover plate rises under the load of the construction material resting thereon. A disadvantage of the adjustable load support described in this German patent is that the material forced into the pressurized compartment distributes itself in a wholly uncontrollable manner between the bottom of the container and the elastomeric mass. When the cover plate is raised to a high elevation, the pressurized medium may sometimes enter the gap between the side walls and the elastomeric mass despite the presence of the sealing elements. It is also possible for the pressurized medium to flow out of this gap to the exterior of the adjustable load support. This is especially likely to occur when the load is especially heavy and the sealing elements are worn away after a number of tilting movements of the cover plate.
This problem is avoided by the adjustable load support which is described in German Patent DE-AS No. 2527128. In the adjustable load support described in this German patent, a membrane bubble made from an elastic material is introduced between the elastomeric mass and the bottom of the container. The membrane bubble is connected to and is contiguous with the inside of the side walls. A pressure line is used to introduce the pressurized medium into the compartment beneath the membrane bubble. In this case, hydraulic oil is used as the preferred pressurized medium.
However, the adjustable load support of DE-AS No. 2527128 is also not free from difficulties. These difficulties arise from the fact that the membrane bubble is contiguous with the side walls of the compartment (and also extend into the sharp corners of the compartment when the compartment is rectangular in shape). For example, when the cover plate is raised, lowered, or tilted, the membrane bubble which is firmly pressed against the side walls of the container cannot follow the movement of the cover upwards without exerting considerable frictional force against the side walls. This will cause the flexible material of the membrane bubble to distend and, because of the friction against the side walls, lead to rapid wear-and-tear. The flexible bubble membrane cannot withstand such stresses, especially the dynamic overstresses which arise from the tilting movements, and which quickly lead to loosening of the bubble membrane from the side walls. When this occurs, the adjustable load support becomes unusable for lifting and lowering heavy loads.
In addition, the adjustable load support described in DE-AS No. 2527128 encounters sealing problems in connection with the pressure line that runs through the bottom of the container.
To remedy these problems, it has been proposed to insert a flexible, elastic membrane bubble between the elastomeric mass and the bottom of the compartment, wherein the membrane bubble is attached to the bottom of the compartment in a non-displaceable, fluid-proof manner. If, now, a pressurized medium, is forced through the bottom of the container below the membrane bubble, the membrane bubble will curve upwards and will also displace the elastomeric mass upwards. Depending on the quantity of pressurized medium which is forced into the container, the cover plate will rise upwards to a certain degree. In order to lower the load, a quantity of pressurized medium is bled out from the compartment. However, this solution is not entirely satisfactory either. An adjustable load support of this nature will place great stress on the bubble membrane as it is forced to distend. Loosening of the bubble membrane from the bottom of the compartment is also quite likely.